The invention relates to a test circuit for comparing source voltages with preselected set points or set ranges and for determination of the presence of deviations between set value and actual value.
A number of applications within the power electronic technology require control circuits wherein the information, whether in a group of rectifiers either none or at least one is passing electric current, or also if either none or at least one is in a conducting state, is ascertained. A system such as a cycloconverter may comprise a full series of groups of rectifying diodes requiring the collection of information as to their electrical state. Generally the desired information is collected via test circuits providing such information in a variety of ways. The main criteria as to suitability of such test circuits include reliability, thermal stability, and, especially when a larger number of test circuits is required, a system which is inexpensive and simple.
Furthermore, it is frequently desirable to determine if each of a number of source voltages is equal to a set voltage or a set range of voltages in each of a variety of components. A reliable information about the current in a switching element and information whether the current amounts to a certain value is obtainable by determining if the voltage drop produced by the element is smaller or larger than the largest possible voltage drop caused by the operating current in a switched-on situation. Procedures of the prior art for determining the voltage drop of a rectifying diode lead, especially for groups of rectifying diodes, to complex arrangements in obtaining the dual information about the presence or absence of a voltage drop, e.g., by applying a rectifier, a series resistance or a Schmitt circuit with its own voltage source at the level of the voltage to be determined.
Informations obtained in this manner about all the voltages to be traced have to be transferred to the voltage level of the control circuits and then a corresponding logical operation has to be performed with them.
A reliable indicator for the presence or absence of current in a thyristor relies on the presence or absence of voltage between gate and cathode. Test circuits based on the input of this voltage are generally very complicated. In particular for a larger number of thyristors this voltage generally switches a transistor circuit with its own current source and the information is transferred to the potential level of the control circuit or a transistor switch is switched with this voltage enabling the passage of an alternating current signal between the coils of two transformers. At least one independent transformer is needed for each thyristor of the group to be controlled.